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There is no clinical evidence that terbinafine will cause a false positive for cocaine on urine drug screening tests. However, if you only started having positive results on screening tests after taking the terbinafine, that would be suspicious that it would be the cause of false positive results in you.
The usual next step would be to perform a confirmatory test, such as gas chromatography/mass spectrometry, or GC/MS. If the GC/MS test is negative for cocaine, that would prove that the screening test is a false positive. The GC/MS does not prove what is causing a false positive, only that it is a false positive. Since false positive results occur on a regular basis, even in someone that is not taking any medicines, the only way to prove that terbinafine is causing the false positive would be to show that the rate of false positives in higher in people taking terbinafine compared to people that are not taking terbinafine.
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